This invention relates to a circuit for producing a periodic, especially parabolic signal at an output terminal, the circuit comprising a squaring stage having an input for receiving a sawtooth-shaped signal having a trace and a retrace, the output signal of the squaring stage having the same frequency as the received sawtooth-shaped signal and having an essentially parabola-shaped variation during the trace period.
Such a circuit is known from the U.S. Pat. No. 4,216,392 and is used, for example, for generating a control signal for an east-west modulator in a picture display arrangement and/or for dynamic focusing in such an arrangement. This control signal can have the field frequency (50 Hz according to the European television standard) or the line frequency (15.625 kHz according to the aforementioned standard) or any other suitable frequency. In the aforesaid patent the advantages of the squaring method used are disclosed, but also the disadvantage of a parabolic interference signal occurring during the retrace period, and a circuit is described therein for reducing the amplitude of this interference signal.
The invention is based on the recognition that not all detrimental effects of the occurring interference signal are obviated in the circuit of U.S. Pat. No. 4,216,392. By supressing this signal or by reducing the amplitude thereof a signal is nevertheless produced which can excite resonant circuits. Such a circuit, for example, is formed by the self-inductances and the capacitances of a line deflection circuit coupled to the east-west modulator and which can have a high quality factor Q, for example, if no high voltage generator is coupled therewith, which generator could have damped oscillations caused by the interference signal. Picture display arrangements comprising such a separate high voltage generator have often been produced of late. Owing to the aforesaid high Q a disturbance occurs on the display screen which may reach as far as one third of the height of the screen, with the lines on the screen varying in width displaying the edges of the image as undulating lines.